O R I G I N A L P A P E R
A green and simple method for preparation of an efficient palladium adsorbent based on cysteine functionalized
2,3-dialdehyde cellulose
Changqing Ruan.Maria Strømme . Jonas Lindh
Received: 8 April 2016 / Accepted: 27 May 2016 / Published online: 6 June 2016 Ó Springer Science+Business Media Dordrecht 2016
Abstract A green and efficient adsorbent for adsorp- tion of palladium ions was prepared from 2,3-dialde- hyde cellulose (DAC) originating from nanocellulose from the green algae Cladophora. The DAC was functionalized with cysteine via reductive amination in a convenient one-pot procedure to provide the adsorbent. The adsorption properties for adsorbing palladium(II) ions, including capacity, adsorption isotherm and kinetics, were studied. The successful reductive amination of cysteine with 2,3-dialdehyde cellulose was confirmed by FT-IR, elemental analysis and XPS. The adsorbent was characterized by SEM, XRD, gas adsorption and TGA. The adsorbent had a high adsorption capacity (130 mg palladium per gram adsorbent) and enabled fast adsorption of palla- dium(II) ions from solution (80 % of maximum capacity reached in 2 h). Adsorbent materials suit- able for both filters (fibrous) and column matrixes (spherical particles) could be obtained in an efficient manner by controlling the degree of oxidation while producing the DAC material.
Keywords 2,3-dialdehyde cellulose Palladium adsorption Cellulose beads Nanocellulose
Introduction
The transition metal palladium is widely used in e.g.
electronic-, pharmaceutical- and chemical industry, due to its specific physical and chemical properties.
The recovery and extraction of palladium from industrial waste has gained considerable interest due to high demand, low natural abundance (Barakat et al.
2006; Liu et al.2003) and the heavy-metal toxic effect palladium exerts to animals, plants and humans (Nolan and Lippard 2008). Furthermore, palladium is a frequently used catalyst for the formation of C–C bonds in the production of pharmaceuticals and is allowed only in sub 5 ppm amounts in pharmaceuti- cals (Products CfPM2002). Therefore, searching for efficient methods for palladium recovery and extrac- tion is important. Further, palladium can be used as a benchmark transition metal and knowledge gained about palladium recovery could likely be useful for developing recovery methods also for other transition metals, e.g. platinum, rhodium and ruthenium.
Some well-known methods including chemical precipitation (Vincent et al. 2007), ion exchange (Anpilogova et al. 2014), solvent extraction (Paiva et al.2014), membrane separation (Li et al.1994) and adsorption (Gupta et al. 2003; Sharma and Rajesh 2014) are employed to recover palladium. Among the above-mentioned methods, adsorption is highly effi- cient and is frequently used. Several studies have investigated palladium ions adsorption using e.g. thiol- C. Ruan M. Strømme J. Lindh (&)
Nanotechnology and Functional Materials, Department of Engineering Sciences, Uppsala University, Box 534, 75121 Uppsala, Sweden
e-mail: jonas.lindh@angstrom.uu.se DOI 10.1007/s10570-016-0976-0
functionalized silica (Lim et al. 2011; Qishu et al.
2012), modified chitin or chitosan (Fujiwara et al.
2007; Wasikiewicz et al. 2007) and functionalized polymers or resins (Awual et al.2013; Jermakowicz- Bartkowiak et al. 2005; Venkatesan et al. 2005).
Among the adsorbents, some are not environmentally benign and rely on a petrol feedstock, whereas others are green, e.g. chitin or chitosan, but suffer from lower adsorption capacity. Furthermore, the ease of extract- ing or desorbing the palladium ions from the adsorbent in order to recycle the precious metal greatly varies between the different materials, where the biopolymers can be simply incinerated at a relatively low temper- ature to obtain the pure palladium, a procedure that is not convenient for e.g. the silica-based adsorbents.
Several aspects are important to consider when developing an efficient material for adsorption.
Besides the obvious advantages of having a large surface area, ample coordination sites, and efficient coordination of the adsorbate, the practical execution of the adsorption is a key aspect e.g. by using filters or columns. For filters, a material with fibrous character is desired as it is beneficial for the strength of filters. A material used as matrix in columns on the other hand should preferably have a spherical morphology to keep backpressure to a minimum.
Cellulose is the most abundant renewable biopolymer in nature and is a promising raw material in terms of cost of production and ease of modification (Klemm et al.
2005). Periodate oxidation of cellulose is a highly specific reaction for converting the 2,3-dihydroxyl groups to 2,3-dialdehyde groups without significant side reactions (Bruneel and Schacht 1993; Maekawa and Koshijima1984). The resulting aldehyde groups can be further transformed to e.g. carboxyl groups (Maekawa and Koshijima 1984; Takaichi et al. 2014), primary alcohols (Casu et al.1985), or imines (Schiff bases) with primary amines (Sabzalian et al.2014), which enables the use of 2,3-dialdehyde cellulose (DAC) as a suitable start- ing material in a wide range of applications e.g. as an adsorbent of heavy metals (Nada and Hassan 2006), DNA (Su et al.2007) or dyes (Kitkulnumchai et al.2008).
Our group has made research efforts towards finding useful applications of Cladophora nanocellu- lose (Carlsson et al.2014;2015; Metreveli et al.2014;
Nyholm et al. 2011; Razaq et al. 2012). The use of Cladophora nanocellulose is well in line with the concept of sustainable industrial scale processing, as the source of this cellulose is abundant and, in
addition, Cladophora algae are typically considered undesirable pollutants, known for forming large, foul- smelling, algal mats and having adverse effects on the aquatic ecosystem as well as harbouring bacterial pathogens (Mihranyan 2011). Furthermore, no large scale applications for this source of cellulose exists so the use of this source is not in conflict with other applications e.g. food production, fuel production or building materials. Recently, we reported a new and convenient method for producing DAC beads by periodate oxidation of Cladophora nanocellulose (Lindh et al. 2014; 2016). The method provides spontaneous bead formation in a one-pot procedure under aqueous conditions at high degrees of oxidation, circumventing the traditional tedious procedure for bead formation typically involving: (1) dissolution of cellulose, (2) shaping of the beads and (3) regeneration of the polysaccharide, where one or more steps typically employ environmentally harmful solvents (Gericke et al.2013). The periodate oxidation can also be controlled to produce materials with fibrous character by performing less extensive periodate oxidation. Based on our work on periodate oxidation of Cladophora cellulose we now introduce a green, renewable and efficient material for adsorption of palladium ions. Different kinds of DAC was prepared by periodate oxidation and were further functionalized via reductive amination with cysteine in a facile one- pot procedure. The obtained materials were used for palladium ion adsorption and were characterized by SEM, XRD, gas adsorption, FTIR, TGA and XPS.
Experimental Materials
Nanocellulose from Cladophora algae was provided by FMC Biopolymer. Sodium metaperiodate (NaIO4), hydroxylamine hydrochloride (NH2OH.HCl), 2-pico- line-borane (C6H7.BH3),L-cysteine and other chemi- cals used were of analytical or reagent grade and were used as received. Deionized water was used through- out the experimental procedures.
Preparation of DAC
The preparation of DAC proceeded according to literature procedure (Lindh et al. 2014). In short,
Cladophora nanocellulose, 20 g in 1.5 L of water was mixed with 134 g of sodium metaperiodate (5 mol per mol of anhydroglucose units) dissolved in the same medium as the cellulose. The periodate-containing reaction mixture was carefully wrapped in aluminum foil to avoid light exposure. The reaction mixture was vigorously stirred at room temperature in the dark for 13 days. Aliquots were withdrawn after 2 and 13 days (750 mL each time) and named 2D-DAC and 13D- DAC, respectively. The withdrawn aliquots were immediately quenched via the addition of ethylene glycol and washed repeatedly with water to provide pure DAC. DAC samples were dried under reduced pressure at 40°C and used in their dry state for further reactions.
Reductive amination of DAC
Dried DAC (2D-DAC or 13D-DAC), 1 g, was added to a 250 mL round-bottom flask, which contained 100 mL methanol. After vigorous stirring, to make the DAC suspend well,L-cysteine (7.57 g, 10 equiv.) and 2-picoline-borane (2.67 g, 4 equiv.) were added successively. The reaction mixture was stirred at room temperature for 24 h. The products, named 2D- DAC-LC and 13D-DAC-LC, were washed with distilled water followed by ethanol and dried under reduced pressure at 40°C for further use.
Scanning electron microscopy
Scanning electron micrographs were taken with a LEO1550 field-emission SEM instrument (Zeiss, Germany) operated at 1 kV with an in-lens secondary electron detector. Samples were mounted on alu- minum stubs by means of double-sided adhesive carbon tape and sputtered with gold/palladium to avoid charging effects.
X-ray diffraction
X-ray diffraction (XRD) analysis was performed with a diffractometer having Bragg-Bretano geometry (Cu Ka radiation; k = 1.54 A˚ ) (D5000, Siemens/Bruker, Germany). The crystallinity index of the nanocellulose and its derivatives was calculated using the Segal empirical equation (Segal et al.1959).
CrI¼I22:5 I18
I22:5
ð1Þ where CrI stands for crystallinity index of cellulose.
I22.5° represents the peak diffraction intensity corre- sponding to crystalline cellulose and I18°is the peak diffraction intensity corresponding to the amorphous sections at 2-Theta, 22.5° and 18°, respectively.
Nitrogen sorption isotherm analysis
Nitrogen gas adsorption and desorption isotherms of Cladophora nanocellulose and its derivatives were performed on an ASAP 2020 instrument (Micromerit- ics, USA). The specific surface area was calculated based on the BET (Brunauer et al. 1938) method during adsorption with the ASAP 2020 software, while the pore size distribution was determined according to the density functional theory (DFT) method using the model for nitrogen at 77 K.
Fourier transform infrared spectroscopy (FTIR) FTIR spectra were recorded on a Tensor 27 FTIR spectrometer (Bruker, Germany). The resolution was set to 4 cm-1 with 64 scans over a range of 4000–400 cm-1. Samples were ground with KBr and prepared into pellets for FTIR measurements.
Thermal gravimetric analysis (TGA)
Samples were analyzed with a TGA/SDTA851 instru- ment (Mettler Toledo, Switzerland), at a temperature range of 25–800°C with a heating rate of 5 °C per minute under air or nitrogen atmosphere.
X-ray photoelectron spectroscopy (XPS)
High-resolution XPS spectra for C, N, O, S, and Pd were performed with an ESCA instrument (Physical Electronics, U.S.) equipped with an Al Ka source (1486.6 eV, 45 W). The analyzer was operated at 23.5 eV pass energy, while electron and Ar? gun neutralization was applied to compensate for surface charging effects. Elemental atomic concentrations and ratios were calculated from the XPS peak areas with the Multipak software (Physical Electronics, U.S.).
Determination of aldehyde content
In order to determine the degree of oxidation (D.O.), elemental analysis was conducted. The DAC samples were transformed to aldoximes via Schiff base reac- tions with hydroxylamine according to a literature procedure (Lindh et al. 2014) and were analyzed for elemental composition (C, H, and N). To a stirred 100 mL RB-flask was added dried DAC (100 mg), 40 mL of acetate buffer (pH 4.5), and 1.65 mL of hydroxylamine solution (aqueous, 50 wt%). The reaction mixture was stirred at room temperature for 24 h. The product was thoroughly washed with water and dried under reduced pressure prior to elemental analysis. The term ‘‘degree of oxidation’’ (D.O.) represents the ratio of 2,3-alcohols in the anhydroglu- cose units that has been transformed into their corresponding aldehydes. The highest degree of oxidation, i.e. 100 %, corresponds to all anhydroglu- cose units being converted to the corresponding non- cyclic 2,3-dialdehyde structures, which would corre- spond to approximately 12.5 mmol of aldehyde groups per gram of cellulose.
Determination of cysteine content
The degree of functionalization, i.e. the degree of cysteine coupled to the DAC, was determined via elemental analysis (C, H, N and S) of the 2D-DAC-LC and 13D-DAC-LC samples. The term ‘‘degree of functionalization’’ represents the ratio of the original 2,3-alcohols in the anhydroglucose units that has been functionalized withL-cysteine via reductive amination of the periodate induced 2,3-dialdehydes. The highest degree of functionalization, i.e. 100 %, corresponds to all of the original 2,3-dialcohols being functionalized withL-cysteine, which would correspond to approx- imately 12.5 mmol of cysteine per gram of cellulose.
Adsorption experiments
A series of Pd(OAc)2 solutions of various initial concentrations (10–600 mg/L of Pd(OAc)2) were prepared by dissolving given amounts of Pd(OAc)2 in acetonitrile. 10 mL aliquots were placed in 15 mL plastic vials and a given amount of adsorbent was added to each vial. Each vial was then sealed and fixed on a tube rotator (VWR Tube Rotator) and rotated for 24 h. The vials were centrifuged at 4700 g for 10 min.
An aliquot of the supernatant was then withdrawn and analyzed by UV–vis spectroscopy (Shimadzu, UV- 1800, Japan). Blank solutions containing equivalent initial concentration of Pd but without addition of adsorbent were subjected to the same procedure.
Adsorption isotherms were carried out at room temperature, 22.5°C.
The adsorption isotherm data were analyzed by both the Langmuir (2) and Freundlich (3) models, which are expressed in linear form as (Freundlich 1906; Langmuir1918),
Ce qe
¼ 1 q0bþ 1
q0
Ce ð2Þ
lnqe¼ lnKFþ1
nlnCe ð3Þ
respectively. Adsorption kinetics for palladium ion adsorption on DAC-based adsorbent was explored in a batch experiment. A number of vials (15 mL) con- taining the same concentration of Pd(OAc)2(100 mg/
L) with the same amount of adsorbent (4 mg) were prepared, and after 2, 4, 8, 15, 30, 60, 120, 240, 360, 480, 1440, 1860 and 4320 min, the suspension in the corresponding vial was filtered through a PTFE (Polytetrafluoroethylene) filter. Adsorption amount was calculated by the difference of the concentration of initial solution of Pd(OAc)2and the filtrate, both of which were measured with UV–vis spectrophotome- ter. Blank controls were taken without addition of adsorbent. All the experiments were performed at room temperature (22.5°C).
Results and discussion
In this study two types of DACs were prepared, differing in the degree of oxidation (D.O.). One type was oxidized to 30 % D.O. by carrying out the oxidation for two days (2D-DAC) and the other had a D.O. of 85 %, which was achieved by performing the oxidation for 13 days (13D-DAC). The 2D-DAC and the 13D-DAC were subjected to reductive amination with L-cysteine using 2-picoline-borane as reducing agent, providing 2D-DAC-LC with a degree of L- cysteine functionalization of 9 % and 13D-DAC-LC with a degree ofL-cysteine functionalization of 49 % (Fig.1). 2-picoline-borane is a non-toxic and efficient reducing agent, which is selective towards imines and
thus allows the direct conversion of carbonyl com- pounds into amines in a one-pot procedure (Cosenza et al.2011; Ruhaak et al.2010; Sato et al.2004). The functionalized DACs contain plentiful coordination sites, which enable efficient adsorption of palladium.
The L-cysteine functionalized DAC materials were then subjected to the adsorption of Pd(II) ions and provided high adsorption capacity. For convenience, the absorbents 2D-DAC-LC and 13D-DAC-LC loaded with palladium were named 2D-DAC-LC-Pd and 13D-DAC-LC-Pd, respectively.
Adsorption isotherm and kinetics
Equilibrium adsorption time is used to evaluate the effect of the contact time between adsorbent and adsorbate (Awual et al. 2013). The results of the relationship between adsorption amount and contact time are shown in Fig.2a. The data indicate that the adsorption of Pd(II) ions was fast and 80 % of
maximum adsorption amount could be reached after 2 h. As the adsorption almost reached equilibrium in 24 h, this time was selected in further experimental work for obtaining the adsorption isotherms.
An adsorption isotherm can be used to characterize the relationship between the adsorbent and the adsor- bate. This provides a relationship between the con- centration of adsorbate in the solution and the amount of adsorbate adsorbed on to solid phase when the two phases are at equilibrium (Sag˘2001). The adsorption isotherms for Pd(II) on 2D-DAC-LC and 13D-DAC- LC are presented in Fig.2b. The amount of metal ions adsorbed at equilibrium per unit mass (qe) of the DAC- LC increased first with the increase of the equilibrium concentration of solute (Ce) then almost reached a plateau value, which represents saturation of the active chelation sites on DAC-LC.
The Langmuir plot (Ce/qeagainst Ce) was used to obtain Langmuir constants, q0 and b, related to the adsorption capacity and energy of adsorption, Fig. 1 Reaction scheme for the preparation of cysteine functionalized DAC and a plausible coordination mode for the adsorption of Pd(II) ions
Fig. 2 a Effect of contact time on Pd(II) adsorption and b adsorption isotherm for Pd(II) on 2D-DAC-LC and 13D-DAC-LC.
Connecting line in a and b only to serve as guide to the eye
respectively. From the Freundlich model, the n and KF are provided, which relates to the adsorption intensity and the adsorption capacity, respectively. All the parameters are shown in Table1. The Langmuir model yielded high correlation coefficient values (R2) 0.9988 and 0.9985 and the q0values were 130.4 mg/g and 60.7 mg/g for Pd(II) adsorbed on 2D-DAC-LC and 13D-DAC-LC, respectively. Values for adsorp- tion capacity of other adsorbents presented in litera- ture are given in Table2 for comparison. The somewhat surprising higher adsorption capacity for 2D-DAC-LC compared to 13D-DAC-LC, despite the lower content of coordinating cysteine groups, is probably a result of lower accessible surface area of the bead shaped 13D-DAC-LC (see Table3).
The thermodynamic parameter, the Gibbs free energy (DG°) (see Table1), for 2D-DAC-LC and 13D-DAC-LC was evaluated from the Langmuir constant using the equation (Gupta et al.2006)
DG¼ RTlnb ð4Þ
where b is the equilibrium constant and R is the universal gas constant. The negative values of DG°
confirm the feasibility of the adsorption process.
In order to investigate the mechanism of adsorp- tion, the kinetic models for pseudo first order (5) (Lagergren1898) and pseudo second order (6) (Sparks 1986) were applied. The pseudo first order model is expressed by
log qð e qtÞ ¼ log qe k1t
2:303 ð5Þ
where qe and qt are the amounts of adsorbed Pd(II) (mg/g) at equilibrium and time t, respectively, and k1 (min-1) is the rate constant of pseudo-first-order adsorption. The pseudo second order model is given by
t qt
¼ 1
k2q2eþ t qe
ð6Þ
where k2[g/(mg min)] is the rate constant of pseudo- second-order adsorption, qe is the amount of Pd(II) adsorbed (mg/g) at equilibrium, and qtis the amount of the adsorption (mg/g) at any time t.
Based on the results in Fig.3, it is obvious that the adsorption behavior is best described by the pseudo second order model, providing a R2value [0.99 for both adsorbents, implying that the adsorption kinetics is largely controlled by chemisorption.
Morphological characterization
SEM micrographs of samples of unmodified Clado- phora nanocellulose, 2D-DAC, 13D-DAC and the corresponding ligand functionalized derivatives, before and after adsorption of palladium, are shown in Fig.4. As previously described (Lindh et al.2014), extensive periodate oxidation, i.e. 13 days oxidation, provided drastic change to the cellulose morphology (producing beads with a relatively smooth surface), while 2 days oxidation induced a more compact cellulose compared to the unmodified Cladophora nanocellulose. Furthermore, the reductive amination withL-cysteine on DAC provided materials with less compact and more porous texture. Adsorption exper- iments provided little or no change to the derivatives morphology, neither for 2D-DAC nor 13D-DAC.
FTIR spectroscopy
Figure5 illustrates the FTIR spectra of unmodified Cladophora nanocellulose and the functionalized derivatives. The signals at 880 (Fig. 5b) and 1734 cm-1 (Fig.5c) are assigned to the hemiacetal and carbonyl groups, respectively (Rowen et al.1951;
Spedding 1960). When studying the spectra of Cladophora cellulose, 2D-DAC and 13D-DAC in these two regions, the signals increase as the D.O.
increases. Furthermore, afterL-cysteine-modification,
Table 1 Adsorption parameters of Pd(II) ions on 2D-DAC-LC and 13D-DAC-LC and Gibbs free energy (DG°) for adsorption
Adsorbent Langmuir Freundlich -DG° (kJ/mol)
q0(mg/g) b 9 104(L/mol) R2 KF n R2
2D-DAC-LC 130.4 3.566 0.9988 82.63 12.17 0.9644 25.75
13D-DAC-LC 60.7 2.496 0.9985 33.20 9.34 0.9722 24.87
a more distinct signal at 1734 cm-1corresponding to carbonyl groups (Spedding 1960) (comparing ii) and iii), or v) and vi)), verified the introduction of
L-cysteine on DAC.
X-ray photoelectron spectroscopy (XPS)
The XPS spectra in wide scans of 2D-DAC, 2D-DAC- LC, 2D-DAC-LC-Pd, 13D-DAC, 13D-DAC-LC and 13D-DAC-LC-Pd are presented in Fig.6. From the spectra of 2D-DAC and 13D-DAC, the characteristic peaks of C 1s (285 eV) and O 1s (531 eV) are shown.
For the spectra of 2D-DAC-LC and 13D-DAC-LC, the additional peaks corresponding to N 1s (398 eV) and S 2p (164 eV) peaks can be clearly seen and the spectra of 2D-DAC-LC-Pd and 13D-DAC-LC-Pd reveal the further addition of signals for Pd 3d5 (335 eV) and Pd 3d3 (340 eV) (Moulder et al.1992).
Table 2 Comparison of adsorption capacity of different adsorbents
Adsorbent Adsorption
capacity (mg/g)
References
Cellulose-MBT 5 (Sharma and Rajesh2014)
Native cellulose 1.87 (Sharma and Rajesh2014)
Radiation cross-linked carboxymethylchitin hydrogels 2.68 (Wasikiewicz et al.2007)
Chitosan resin modified withL-lysine 109.47 (Fujiwara et al.2007)
Polystyrene-divinylbenzene functionalized with isothiouronium 20 (Venkatesan et al.2005)
Carbon black modified withL-Cysteine 84 (Panchompoo et al.2011)
Mesoporous silica functionalized with 3-(((5-ethoxybenzenethiol) imino)methyl)-salicylic acid ligand
164.2 (Awual et al.2013)
2D-DAC-LC 130.4 This work
13D-DAC-LC 60.7 This work
Table 3 Pore volume and SSA of cellulose and its derivatives Pore volume (cm3/g) SSA (m2/g)
Cladophora cellulose 0.6 102
2D-DAC 0.4 47
2D-DAC-LC 1 141
13D-DAC 0.003 1
13D-DAC-LC 0.3 47
Fig. 3 aPseudo first order kinetics and b pseudo second order kinetics for Pd(II) on 2D-DAC-LC and 13D-DAC-LC (the concentration of palladium acetate in acetonitrile was 100 mg/L)
Thermal gravimetric analysis (TGA)
Thermal stability of nanocellulose, oxidized nanocel- lulose and the corresponding derivatives was investi- gated by TGA and the obtained curves are depicted in Fig.7. It can be seen that degradation of 2D-DAC and 13D-DAC compared to that of unmodified Clado- phora nanocellulose is initiated at a lower tempera- ture, both in compressed air and under nitrogen atmosphere. Furthermore, the degradation of 2D- DAC-LC and 13D-DAC-LC commences at a lower temperature than that of 2D-DAC and 13D-DAC, which indicates that the thermal stability of materials deriving from nanocellulose decreases after modifica- tion in terms of oxidation andL-cysteine modification.
On the other hand, final weight percentages of 2D- DAC-LC-Pd and 13D-DAC-LC-Pd are substantially higher than for the corresponding material before
adsorption of palladium, both in compressed air and nitrogen atmosphere, which confirms the adsorption of palladium ions during the adsorption experiments.
These TGA results confirm the XPS results that palladium ions were adsorbed on 2D-DAC-LC and 13D-DAC-LC and also demonstrate that the
L-cysteine modification was functional.
Surface area and porosity analysis
Nitrogen sorption isotherms at a temperature of 77 K and pore size distribution for Cladophora nanocellu- lose and its derivatives are given in Fig.8. The pore volume and specific surface area (SSA) of Cladophora nanocellulose and its derivatives are displayed in Table 3. The SSA decreases with increased D.O., two orders of magnitude from 102 to 1 m2/g corresponding to unmodified Cladophora nanocellulose and 13D- Fig. 4 SEM micrographs of a unmodified Cladophora nanocellulose, b 2D-DAC, c 2D-DAC-LC, d 2D-DAC-LC-Pd, e 13D-DAC, f13D-DAC-LC and g 13D-DAC-LC-Pd
DAC. Interestingly, after the modification with
L-cysteine, the SSA increases significantly, to 141 m2/g for 2D-DAC-LC and to 47 m2/g for 13D- DAC-LC, which is supported by the more porous texture observed by SEM (see Fig.4). In Fig. 8b, the pore size distributions of the samples are shown. All of the samples have a pore size range primarily in the mesoporous region (2–50 nm), except for 2D-DAC-
LC, which has a broader pore size distribution with a mode at 45 nm.
X-ray diffraction (XRD) spectroscopy
XRD curves for Cladophora cellulose, 2D-DAC, 2D- DAC-LC, 2D-DAC-LC-Pd, 13D-DAC, 13D-DAC- LC and 13D-DAC-LC-Pd are given in Fig.9. As Fig. 5 FT-IR spectra of a 4000–400 cm-1, b 1000–800 cm-1and c 1850–1650 cm-1for i) unmodified Cladophora nanocellulose, ii) 2D-DAC, iii) 2D-DAC-LC, iv) 2D-DAC-LC-Pd, v) 13D-DAC, vi) 13D-DAC-LC and vii) 13D-DAC-LC-Pd
Fig. 6 XPS wide scans of a 0–1100 eV, b 150–180 eV and c 345–405 eV for i) 2D-DAC, ii) 2D-DAC-LC, iii) 2D-DAC-LC-Pd, iv) 13D-DAC, v) 13D-DAC-LC and vi) 13D-DAC-LC-Pd
described in Segal’s empirical equation (Segal et al.
1959), the intensity of 2-Theta at about 22.5° and 18°
corresponds to crystalline and amorphous sections of cellulose, respectively. The intensity of peaks at about 22.5° decreases for 2D-DAC, 2D-DAC-LC and 2D- DAC-LC-Pd compared to unmodified cellulose, which indicates a decrease of crystallinity of the cellulose derivatives after chemical functionalization. Further- more, the peaks at about 22.5° disappear in the spectra of 13D-DAC, 13D-DAC-LC and 13D-DAC-LC-Pd, which suggest that these samples become completely amorphous.
Conclusions
The novel adsorption materials described herein provide a green and efficient alternative to frequently used adsorbents based on a petrol feedstock. The use of environmentally benign and renewable cellulose from the undesirable polluting green algae Cladophora, functionalized with the natural amino acid cysteine, constitutes a green approach to enable recovery of the transition metal palladium. The results from the adsorption study indicate that the adsorption of palla- dium ions is mainly governed by chemisorption. The Fig. 7 Thermograms performed in a compressed air and b nitrogen for dried Cladophora nanocellulose, 2D-DAC, 2D-DAC-LC, 2D- DAC-LC-Pd, 13D-DAC, 13D-DAC-LC and 13D-DAC-LC-Pd
Fig. 8 aNitrogen sorption isotherms (T = 77 K) and b pore size distribution for cellulose and its derivatives
adsorbent has a high adsorption capacity and enables fast adsorption of palladium ions from solution. With our process, materials suitable for both filters (fibrous materials) and for column matrixes (spherical beads) can be obtained. The adsorption materials were extensively characterized using several different meth- ods e.g. FTIR, SEM, XRD, XPS, TGA, elemental analysis and gas adsorption.
Acknowledgments Ollie and Elof Ericsson´s Foundation as well as the Bo Rydin Foundation are gratefully acknowledged for their financial support. Changqing Ruan thanks the China Scholarship Council (CSC) for financial support.
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